In the field of metal casting, solute composition inhomogeneities at the macroscale are called macrosegregation, and the transition from the elongated grains in the outer portions of a casting to the more rounded grains in the center is termed Columnar to Equiaxed Transition (CET). Simultaneous prediction of macrosegregation and CET is still an important challenge in the field. One of the open questions is the role of melt convection on the CET and the effect of the CET on macrosegregation. A three-phase macroscale model for macrosegregation and CET was developed. The model accounts for numerous phenomena such as columnar dendrite tip undercooling, undercooling behind the columnar tips, and nucleation of equiaxed grains. This three-phase model was used to develop a less complex model that consists of two phases only and disregards undercooling behind the columnar tips and nucleation of equiaxed grains. An in-house parallel computing code on the OpenFOAM platform was developed to solve the equations of these models. The models were used to perform columnar solidification simulations of a numerical benchmark problem. It was found that the predictions of these models are nearly identical. It was also found that the dendrite tip selection parameter, which appears in the constitutive relation for the dendrite tip velocity, plays a key role in these models. With a realistic value for this parameter these models account for columnar dendrite tip undercooling, but as its value is increased in the simulations, predictions of these models converge to predictions of a model that neglects undercooling. Next, the three-phase model was used to perform CET simulations in the numerical solidification benchmark problem in the presence of melt convection. It was found that accounting for stationary equiaxed grains does not change the overall macrosegregation pattern nor the form of channel segregates. Finally, for the first time in the field of solidification, we developed accurate constitutive relations for macroscale solidification models that are based on a formal mesoscale analysis on the scale of a representative elementary volume that is used in developing volume-averaged macroscale models. This upscaling enabled us to present relations that incorporate changes in the shape of grains and solute diffusion conditions around them during growth. The models and constitutive relations we developed can now be used to predict critical phenomena such as macrosegregation, channel segregates, and CET in castings.

Impaired mass transfer characteristics of blood borne vasoactive species such as ATP in regions such as an arterial bifurcation have been hypothesized as a prospective mechanism in the aetiology of atherosclerotic lesions. Arterial endothelial (EC) and smooth muscle cells (SMC) respond differentially to altered local hemodynamics and produce coordinated macro-scale responses via intercellular communication. Using a computationally designed arterial segment comprising large populations of mathematically modelled coupled ECs & SMCs, we investigate their response to spatial gradients of blood borne agonist concentrations and the effect of micro-scale driven perturbation on the macro-scale. Altering homocellular (between same cell type) and heterocellular (between different cell types) intercellular coupling we simulated four cases of normal and pathological arterial segments experiencing an identical gradient in the concentration of the agonist. Results show that the heterocellular calcium (Ca2+) coupling between ECs and SMCs is important in eliciting a rapid response when the vessel segment is stimulated by the agonist gradient. In the absence of heterocellular coupling, homocellular Ca2+ coupling between smooth muscle cells is necessary for propagation of Ca2+ waves from downstream to upstream cells axially. Desynchronized intracellular Ca2+ oscillations in coupled smooth muscle cells are mandatory for this propagation. Upon decoupling the heterocellular membrane potential, the arterial segment looses the inhibitory effect of endothelial cells on the Ca2+ dynamics of underlying smooth muscle cells. The full system comprising hundreds of thousands of coupled nonlinear ordinary differential equations simulated on the massively parallel Blue Gene architecture. The use of massively parallel computational architectures shows the capability of this approach to address macro-scale phenomena driven by elementary micro-scale components of the system.

The work investigates the reduction of friction between non-conformal macro contacts with partial slip. The measurement of the coefficient of friction, with the magnitude of the superlubricity, is extended by the influence of relative slip, direct observation of the contact area and the measurement of the thickness of the lubricating film. It also includes a comparison of changes in topography by friction. The experiments were mainly performed on a ball-on-disk tribometer with measuring the film thickness by colorimetic interferometry. The surface roughness changes were investigated using an optical profilometer. The effect of mean velocity, slide-to-roll radio and topography were observed for selected combinations, one of which did not allow the measurement of film thickness and one did not achieve the expected reduction in friction. For the other combinations, it was possible to observe a reduction of the coefficient of friction to 0,003 or less. The influences affecting the friction between selected materials have been investigated considerably. Thus, a foundation is laid for the research of superlubricity between contacts with partial rolling. The thesis could contribute to the reduction of friction losses, earlier(quicker/faster?) and more common use of some lubricants and materials in mechanical engineering.

Surfactants are amphiphilic molecules which self-assemble at the interface in oil-water-surfactant mixtures such that the hydrophobic part, called tail, stays in oil and the remaining part, called head, resides in hydrophilic en-vironment. Depending upon concentration of individual components, these mixtures form several microphases, such as bilayers, micelles, columnar and lamellar phases. A lamellar phase, at equilibrium, is made up of alternat-ing layers of water and oil separated by surfactants, or of alternate layers of water and surfactant bilayers such that the hydrophilic heads are in contact with water. This equilibrium state is rarely achieved in macroscopic samples due to thermodynamic and kinetic constraints; instead, a lamellar fluid is usually disordered with a large number of defects. These defects have significant effect on the flow behaviour of the lamellar mesophase systems. They are known to alter the flow field, resulting stresses and in turn could get distorted or annihilated by the flow. In present work, we analyse this two way coupling between lamellar structure and flow field. The structural and rheological evolution of an initially disordered lamellar phase system under a shear flow is examined using a mesoscale model based on a free energy functional for the concentration field, which is the scaled difference in the concentration between the hydrophilic and hydrophobic components. Two distinct modes of structural evolution are observed depending only on Peclet number, which ratio of inertial forces to mass diffusivity, in-dependent of system size. At low Peclet number, local domains are formed which are then rotated and stretched by shear. A balance between defect creation and annihilation is reached due to which the system never reaches the equilibrium layer configuration. In the opposite limit, partially formed layers break and reform so as to form a nearly aligned lamellar phase con-figuration with residual defects. Viscosity of lamellar phase system increases with layer moduli, differences in viscosity of individual components, fluidity of the lamellae due to shear banding and defect pinning. These factors however, do not have any effect on alignment mechanism.